1. Field of the Invention
This invention relates generally to the backplate of a field emission display, and more particularly to a self aligned focusing grid for field emitters that emit electrons to corresponding phosphor pixels. Further, this invention relates to a locator formed on an interior surface of the backplate for receiving a spacer wall.
2. Description of the Related Art
Field emission devices include a faceplate, a backplate and connecting walls around the periphery of the faceplate and backplate, forming a sealed vacuum envelope. Generally in field emission devices, the envelope is held at vacuum pressure, which in the case of CRT displays is about 1.times.10.sup.-7 torr or less. The interior surface of the faceplate is coated with light emissive elements, such as phosphor or phosphor patterns, which define an active region of the display. Cathodes, (field emitters) located adjacent to the backplate, are excited to release electrons which are accelerated toward the phosphor on the faceplate, striking the phosphor, and causing the phosphor to emit light seen by the viewer at the exterior of the faceplate. Emitted electrons for each of the sets of the cathodes are intended to strike only certain targeted phosphors. There is generally a one-to-one correspondence between each emitter and a phosphor.
Flat panel displays are used in applications where the form-factor of a flat display is required. These applications are typically where there are weight constraints and the space available for installation is limited, such as in aircraft or portable computers.
A certain level of color purity and contrast are needed in field emission devices. Contrast is the difference between dark and bright areas. The higher the contrast, the better. The parameters of resolution, color-purity and contrast in a flat cathode luminescent display depend on the precise communication of a selected electron emitter with its corresponding phosphor pixels. Additionally, high picture brightness (lumens), requires either high power consumption or high phosphor efficiency (lumens/watt).
High power consumption in many applications is not desirable. Efficiency for many phosphors increases as the operating anode voltage increases; and the required operating brightness can be achieved with lower power consumption at high voltage, as illustrated in FIG. 1. In order to satisfactorily operate at high anode voltages, e.g., 4 kV or higher, the backplate containing the emitter array must be spatially separated from the faceplate, containing the phosphor pixels, by a distance sufficient to prevent unwanted electrical events between the two. This distance, depending on the quality of the vacuum and the topography of the substrates, is typically greater than about 2 mm.
With the constraints of faceplate and backplate glass area and thickness, the vacuum envelope is unable to withstand 1 atmosphere or greater external pressure without inclusion of the spacer walls. If the spacer walls are not included then the faceplate and backplate can collapse. In rectangular displays, having greater than approximately a 1 inch diagonal, the faceplate and backplate are particularly susceptible to this type of mechanical failure due to their high aspect ratio, which is defined as the larger dimension of the display divided by the thickness of the faceplate or backplate. The use of spacer walls in the interior of the field emission device substantially eliminates this mechanical failure.
The use of spacer walls has been reported in U.S. Pat. No. 4,900,981; U.S. Pat. No. 5,170,100; EPO 464 938 A1; EPO 436 997 A1; EPO 580 244 A1; and EPO 496 450 A1.
The faceplates and backplates for the desired flat, light portable display are typically about 1 mm thick. To avoid seeing the spacer walls at the exterior of the faceplate, the spacer walls should be hidden behind a suitable structure such as a black matrix.
The angular distribution of electrons from certain types of electron emitters is such that there is substantial emission at field emitter cone half angles greater than about 45 degrees. In devices where the electron emitter is located 2 mm from the corresponding picture element, the projection electrons from emitter will illuminate a disc with an area greater than 4 mm in diameter.
A ten inch diagonal color display used in portable computers, at VGA color resolution requires that the area illuminated by each electron emission source not exceed 0.00417 inches in diameter to maintain purity of color. In these high energy phosphor displays it is necessary to restrict and focus the electron beam that is generated. For this VGA display, the maximum locational tolerance for the position of the electron beam at the picture element is 0.0005 inches. This is one-half the width of a column guard band in the black matrix surrounding each color sub-pixel.
The total tolerance budget for location of the electron beam relative to its corresponding pixel is the summation of positional errors in the geometrical alignment of the substrate containing the electron emitters to the faceplate containing the phosphor sub-pixels.
Of the phosphor to black matrix, and the field emitter to focus alignment, the latter is the most critical because deflection of the electron beam by the focus grid is a function of the electric field generated by the focus grid. The electron-optical properties of the focus grid are such that any misalignment of the emitters in the focus grid will be amplified, as seen in the position of the electron beam on the phosphor coated faceplate.
It would be desirable to minimize misalignment of the electron beam and the consequential loss of color purity and make the principal axis of the electron beam coaxial with the focusing lens. It would also be desirable to create a focus electrode that is self aligned to the field emitter. It would be further desirable to provide a self aligned focus grid for a field emission display.